Over the years, Nd—Fe—B sintered magnets find an ever increasing range of application including electric appliances, industrial equipment, electric vehicles and wind power plants. It is required to further improve the performance of Nd—Fe—B magnets.
A variety of approaches were taken for improving properties of Nd—Fe—B sintered magnets. Approaches for improving coercive force include refinement of grains, addition of Al, Ga or similar elements, and increase in the volume fraction of Nd-rich phase. The currently most common approach is substitution of Dy or Tb for part of Nd.
It is believed that the coercivity creating mechanism of Nd—Fe—B magnets is the nucleation type wherein nucleation of reverse magnetic domains at grain boundaries of R2Fe14B major phase governs a coercive force. Substituting Dy or Tb for some Nd increases the anisotropic magnetic field of the R2Fe14B phase to prevent nucleation of reverse magnetic domains whereby the coercive force is increased. When Dy or Tb is added in an ordinary way, however, a loss of remanence (or residual magnetic flux density) is unavoidable because Dy or Tb substitution occurs not only near the interface of major phase grains, but even in the interior of the grains. Another problem is an increased amount of expensive Tb and Dy used.
Also developed was a two-alloy method of preparing an Nd—Fe—B magnet by mixing two powdered alloys of different composition and sintering the mixture.
Specifically, a powder of alloy composed mainly of R2Fe14B phase wherein R is Nd and Pr is mixed with a powder of R-rich alloy containing Dy or Tb. This is followed by fine pulverization, compaction in a magnetic field, sintering, and aging treatment whereby the Nd—Fe—B magnet is prepared (see JP-B H05-031807 and JP-A H05-021218). The sintered magnet thus obtained produces a high coercive force while minimizing a decline of remanence because Dy or Tb substitutes only near the grain boundary having a substantial impact on coercive force, and Nd or Pr in the grain interior is kept intact. In this method, however, Dy or Tb diffuses into the interior of major phase grains during the sintering so that the layer where Dy or Tb is segregated near grain boundaries has a thickness equal to or more than about 1 micrometer, which is substantially greater than the depth where nucleation of reverse magnetic domains occurs. The results are still unsatisfactory.
Recently, there were developed several processes of diffusing rare earth elements from the surface to the interior of a mother R—Fe—B sintered body. In one exemplary process, a rare earth metal such as Yb, Dy, Pr or Tb, or Al or Ta is deposited on the surface of Nd—Fe—B magnet using an evaporation or sputtering technique, followed by heat treatment. See JP-A S62-074048, JP-A H01-117303, JP-A 2004-296973, JP-A 2004-304038, JP-A 2005-011973; K. T. Park, K. Hiraga and M. Sagawa, “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets,” Proceedings of the Sixteen International Workshop on Rare-Earth Magnets and Their Applications, Sendai, p. 257 (2000); and K. Machida and T. Lie, “High-Performance Rare Earth Magnet Having Specific Element Segregated at Grain Boundaries,” Metal, 78, 760 (2008). In addition, diffusion of Dy from the surface of a sintered body in Dy vapor atmosphere is described in WO 2007/102391 and WO 2008/023731. A process involving coating a powder of rare earth inorganic compound such as fluoride or oxide onto the surface of a sintered body and heat treatment is described in WO 2006/043348. Diffusion of rare earth is effected while rare earth fluoride or oxide is chemically reduced with a CaH2 reducing agent as disclosed in WO 2006/064848. Use of rare earth-containing intermetallic compound powder is disclosed in JP-A 2008-263179.
With these processes, the elements (e.g., Dy and Tb) disposed on the surface of the mother sintered body travel mainly along grain boundaries in the sintered body structure and diffuse into the interior of the mother sintered body during the heat treatment. If heat treatment conditions are optimized, there is obtained a structure in which the lattice diffusion into the major phase grain interior is restrained, and Dy and Tb are enriched in a very high concentration only at grain boundaries or near grain boundaries within sintered body major phase grains. As compared with the two-alloy method described previously, this structure provides an ideal morphology. Since the magnetic properties reflect the morphology, the magnet produces a minimized decline of remanence and an increased coercive force, accomplishing a drastic improvement in magnet performance.
However, the processes utilizing evaporation or sputtering (described in JP-A S62-074048, JP-A H01-117303, JP-A 2004-296973, JP-A 2004-304038, JP-A 2005-011973, WO 2007/102391, WO 2008/023731, and the article of Park, et al.) are problematic in mass production because treatment of a large amount of material at a time is difficult and magnet properties vary over a wide range. The process also suffers from a substantial loss of Dy since most of Dy evaporating from the source scatters in the chamber.
The process described in WO 2006/064848 relies on the chemical reduction of rare earth fluorides or oxides with a CaH2 reducing agent. It is also unamenable to mass production because CaH2 is readily reactive with moisture and hazardous to handle.
In the process of JP-A 2008-263179, a sintered body is coated with a powder composed mainly of an intermetallic compound phase consisting of a rare earth element such as Dy or Tb and an element M which is selected from Al, Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb, and Bi and mixtures thereof, followed by heat treatment. The process has the advantage of easy handling because the intermetallic compound is hard and brittle and thus easy to pulverize, and less susceptible to oxidation or reaction even when dispersed in liquids such as water and alcohols. However, the intermetallic compound is not completely unsusceptible to oxidation or reaction. If deviated from the desired composition, some reactive phases other than the intermetallic compound phase may form, which are prone to ignition and combustion.